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Mixing C and Assembly

Hi all

Pardon me for my ignorance as I am new to assembly programming. Would like to seek help in this problem of mine.

I have a function which needs to be called in C and implemented in assembly. In C, the function is called "char_put(&c)" where a pointer is passed in.

1) How do I access this pointer in assembly?

2) How do I store data into the memory location pointed to by the
pointer? (Suppose to be a 8bit data)

3) When completed, the procedure needs to return a "1" if data is
store or "0" if otherwise. How do I return a value to C from

Thank you so much in advance. Any advice is appreciated.

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1 Solution
In the standard C calling convention, all variables are passed on the stack and the callee can return a value in eax.

This is how the stack looks in the callee:

return address

For example (nasm)

  mov edx,[esp + 4] ; load first argument from stack (a string pointer)
  movzx eax,byte [edx] ; load and zero extend first character from the string
  mov byte [edx],0 ; truncate the string
  ret ; return from function, result is in eax

Note that the callee may clobber eax, ecx and edx.

Here's the C equivalent of the above assembly function:

int char_put(char *s)
  int first;

  first = (int) s[0];
  s[0] = 0;
  return first;

You can also write the wanted function in C, let the compiler generate the assembly and then modify the generated function.
WtwhAuthor Commented:
Thank you for the response. It was most useful.

I have 2 more queries.

1) The parameter passed in from the C function is a pointer which is supposed to be modified by the assembly procedure. How do I change the value of the data pointed to by the pointer? ie How do I access the memory location when the argument passed in is the pointer?

2) Which compiler can I use which can generate the assembly code from C function?

Apologise for my ignorance again as I'm really new to all these.

You load the argument (a pointer) from the stack into a register and then you can dereference that register. The example including its C equivalent:

mov edx,[esp + 4] ; edx = s
movzx eax,byte [edx] ; eax = (int) *s;
mov byte [edx],0 ; *s = 0;

If you're completely new to assembly, it's easiest to learn the basics of assembly first and then try to hook it to C code. The latter is quite easy actually. You only have to respect the calling convention and then assembly your sources into an object file that can be linked as usual.

Every compiler that I know of can generate assembly. In fact, most consist of a separate compiler, assembler and linker. For example gcc has the -S switch to output the assembly of a C/C++ source. MSVC has a similar switch.

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